The modified temperature term on Johnson Cook model for AZ31 magnesium alloy

The quasi‐state and dynamic mechanism of AZ31 magnesium alloy at a strain rates range of 0.001 s^‐1–2500 s^‐1 under a temperature range of 20 °C–250 °C were researched by compression tests using the electronic universal testing machine and split Hopkinson pressure bar system. The true stress‐strain curves at different strain rates and evaluated temperatures were obtained. The result shows that the thermal soften effect of AZ31 magnesium alloy is significant. By modifying the temperature term of the original Johnson Cook model of AZ31 magnesium alloy, a modified Johnson Cook model of AZ31 magnesium alloy has been proposed to reveal thermal soften effect on the deformation behavior of AZ31 magnesium alloy more precisely. With the modified Johnson Cook model and fracture model, the finite element method simulation of AZ31 magnesium alloy hat shaped specimen under impacting was conducted. The numerical simulation result is consistent with the experimental result, which indicates that the modified Johnson Cook model and fracture model are greatly valid to predict the deformation and fracture behavior of the AZ31 magnesium alloy hat shaped specimen under impacting.     

异步轧制AZ31镁合金板材的超塑性工艺及变形机制

经过异步轧制工艺获得AZ31镁合金薄板。在300~450℃范围内,分别通过5×10-3,1×10-3s-1和5×10-4s-1不同应变速率进行高温拉伸实验研究其超塑性变形行为,计算应变速率敏感指数m值、超塑性变形激活能Q及门槛应力σ0值。通过EBSD分析和扫描电镜观察拉伸断裂后的断口形貌,分析AZ31镁合金的超塑性变形机制。结果表明:AZ31镁合金的塑性变形能力随着变形温度的升高及应变速率的降低而增强。当拉伸温度为400℃、m=0.72、应变速率为5×10-4s-1时,AZ31具有良好的超塑性,伸长率最大为206%。温度为400℃时,异步轧制AZ31镁合金的超塑性变形是以晶格扩散控制的晶界滑移和基面滑移共同完成的。     

Modeling high-temperature tensile deformation behavior of AZ31B magnesium alloy considering strain effects

The hot tensile deformation behaviors of AZ31B magnesium alloy are investigated over wide ranges of forming temperature and strain rate. Considering the effects of strain on material constants, a comprehensive constitutive model is applied to describe the relationships of flow stress, strain rate and forming temperature for AZ31B magnesium alloy. The results show that: (1) The effects of forming temperature and strain rate on the flow behaviors of AZ31B magnesium alloy are significant. The true stress–true strain curves exhibit a peak stress at small strains, after which the flow stress decreases until large strain, showing an obvious dynamic softening behavior. A considerable strain hardening stage with a uniform macroscopic deformation appears under the temperatures of 523 and 573 K. The strain hardening exponent (n) increases with the increase of strain rate or the decrease of forming temperature. There are not obvious strain-hardening stages when the forming temperature is relatively high, which indicates that the dynamic recrystallization (DRX) occurs under the high forming temperature, and the balance of strain hardening and DRX softening is easy to obtain. (2) The predicted stress–strain values by the established model well agree with experimental results, which confirm that the established constitutive equation can give an accurate and precise estimate of the flow stress for AZ31B magnesium alloy.     

加工方式对AZ31变形镁合金热拉伸流变应力的影响

针对不同方法制备的AZ31镁合金薄板,利用热拉伸试验机和金相显微镜对其在不同温度和变形速率下的流变应力进行了实验研究.结果表明:挤压、交叉、热轧和冷轧等方法制备的AZ31镁合金薄板的应力-应变曲线基本特征是相同的.峰值流变应力随变形温度的升高和应变速率的降低而降低,在低温时具有明显的厚度效应;当温度大于350℃时峰值流变应力几乎不随板材厚度变化而变化;应变速率小于1.0×10-2s-1,变形温度大于150℃下所有AZ31薄板的延伸率均δ≥45%;单向轧制薄板的各向异性随温度提高减小.     

半固态AZ91D镁合金的压缩变形和显微组织

利用平行板触变压缩仪研究了电磁搅拌的半固态AZ91D合金试样的压缩变形和组织.结果表明:随着半固态压缩变形温度的升高,AZ91D镁合金试样变形的速度加快,即变形应变速度增大,但压缩应力不断下降;在某一载荷下,AZ91D镁合金试样压缩变形应力和应变呈明显的线性关系,与压缩温度的高低无关.随着半固态压缩载荷的提高,AZ91D镁合金试样变形的速度增加,应变速度增大,应力下降速度加快;在不同的压缩载荷下,AZ91D镁合金试样的压缩变形应力和应变都呈明显的线性关系.在实验中的各种半固态压缩变形条件下,初生α-Mg在压缩后AZ91D镁合金试样组织中的分布很均匀,几乎不存在组织偏析.当初生固相的形态呈球状结构,在相同的变形条件下,不同种类合金的半固态压缩变形规律非常相似.     

Hot working behavior of AZ31 and ME21 magnesium alloys

The plastic deformation and recrystallization behavior of the commercial magnesium alloys AZ31 and ME21 were analyzed in a wide temperature range. Using the conventional hyperbolic sine equation the flow stress dependence on temperature and strain rate was modeled. The activation energy for plastic deformation significantly increased with increasing temperature and delivered values above 180 kJmol^?1 for both alloys in the very high-temperature regime (400–550 °C). At lower temperatures (250–400 °C) the activation energy of the AZ31 alloy was approximately 108 kJmol^?1 considering the peak stress as well as 120 kJmol^?1 considering the flow stress at a strain of 0.5. The stress exponent varied in a range between 4.5 and 6.5. During the high-temperature compression tests a partial recrystallized microstructure was formed, which was distinctly different in AZ31 compared to ME21 due to the different onset of dynamic recrystallization (DRX) mechanisms.     

Formability and numerical simulation of AZ31B magnesium alloy sheet in warm stamping process

The potential process for mass production of magnesium alloy components in vehicles—warm stamping process was investigated systematically in the present study. For analyzing the forming process, an accurate numerical model describing the unique characteristics of magnesium alloy sheets under warm forming is very essential. Aiming at this, hardening/softening model for 1.5 mm thickness AZ31B magnesium alloy sheet were firstly constructed based on uniaxial tensile tests. Secondly, semispherical drawing was carried out under the selected temperature to generate experimental forming limit curve (FLC) for AZ31B sheet. Then, friction coefficient was identified using a high-temperature tribo-tester. Finally, numerical simulation was implemented and formability of AZ31B sheet warm forming was verified with experiment. The result shows that the formability, thickness distribution and equivalent strain distribution in simulation agreed well with the actual specimens, which thus provided a good data base for describing the unique characteristics of magnesium alloy sheets under warm forming.     

Microstructure and mechanical behavior of ECAP processed AZ31B over a wide range of loading rates under compression and tension

In this work, a commercial magnesium alloy, AZ31B in hot-rolled condition, has been subjected to severe plastic deformation via four passes of equal channel angular pressing (ECAP) to modify its microstructure. Electron backscatter diffraction (EBSD) was used to characterize the microstructure of the as-received, ECAPed and mechanically loaded specimens. Mechanical properties of the specimens were evaluated under both compression and tension along the rolling/extrusion direction over a wide range of strain rates. The yield strength, ultimate strength and failure strain/elongation under compression and tension were compared in detail to sort out the effects of factors in terms of microstructure and loading conditions. The results show that both the as-received alloy and ECAPed alloy are nearly insensitive to strain rate under compression, and the stress–strain curves exhibit clear sigmoidal shape, pointing to dominance of mechanical twinning responsible for the plastic deformation under compression. All compressive samples fail prematurely via adiabatic shear banding followed by cracking. Significant grain size refinement is identified in the vicinity of the shear crack. Under tension, the yield strength is much higher, with strong rate dependence and much improved tensile ductility in the ECAPed specimens. Tensile ductility is even much larger than the malleability under compression. This supports the operation of 〈c + a〉 dislocations. However, ECAP lowers the yield and flow strengths of the alloy under tension. We attempted to employ a mechanistic model to provide an explanation for the experimental results of plastic deformation and failure, which is in accordance with the physical processes under tension and compression.     

Effect of extrusion ratio on mechanical and corrosion properties of AZ31B alloys prepared by a solid recycling process

Some AZ31B magnesium alloy bars were prepared by a solid recycling process with different extrusion ratios. A reference specimen was processed by extruding an as-received AZ31 ingot. The microstructures, mechanical and corrosion properties of AZ31B magnesium recycled specimens were investigated. With increasing extrusion ratio, the yield strength, tensile strength and yield ratio increases. The reliability of the recycled alloy is poorer than the reference specimen. The corrosion rates of recycled AZ31B magnesium specimens increase immersed in both alkaline and neutral 4% NaCl solution with a decrease extrusion ratio. The corrosion resistance of recycled AZ31B magnesium specimens is improved with increasing pH of immersed solution. The recycled specimens show superior corrosion resistance than reference specimen.     

室温多向压缩道次变形量对AZ80镁合金力学性能影响

目的 探明室温塑性变形对AZ80塑性、硬度及最大应力等力学性能的影响规律,为其成形工艺参数制定提供依据。方法 对挤压态AZ80镁合金均匀化处理后,在室温下控制道次变形量(0.05、0.075、0.1)及累积应变进行多向多道次压缩变形;利用力学试验机和维氏硬度计分析道次变形量与累积应变对其力学性能的影响。结果 在室温下,当AZ80镁合金单向压缩的真应变达到0.124时会发生开裂,通过小应变多向多道次压缩可以将累积应变至少提高至3.6以上。在道次变形量为0.05、0.075和0.1时,累积应变分别可达到7.5、6和3.7;在累积应变为3.6时,随着道次变形量的增加试样硬度(HV)分别达到94、110和121,较未变形试样硬度(70HV)分别提升了33%、57%和73%。结论 AZ80镁合金通过室温多向多道次压缩有利于改善材料塑性,提高力学性能。其塑性随着道次变形量的减小而提高,硬度和最大应力随道次变形量和累积应变的增加而升高,且道次变形量比累积应变对硬度和最大应力的影响更大。     

Zwillingsbildung bei der thermischen Ermüdung der Mg‐Basislegierung AZ31

Twinning at thermal fatigue of magnesium alloy AZ31 In this paper results of thermal fatigue tests of the magnesium base alloy AZ31 carried out in a temperature range between ‐50 °C and +290 °C are presented. Specimens were loaded under constant total strain and uniaxial homogeneous stresses. The resulting materials behaviour is described by stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. It is well known that AZ31 shows different stress‐strain behaviour during tensile and compressive loading resp. at lower temperatures due to the fact that mechanical twinning depends on the loading direction. However untwinning processes may occur during unloading and reloading in the opposite direction. As a consequence, during the first thermal loading cycles, typical consequences of the formation and the dissolution of twins are observed. The interaction of deformation, recovery and recrystallization processes, characteristic for individual temperature ranges are discussed in detail to analyze the damage progress during thermal fatigue.     

Evaluation of fatigue life of AZ31 magnesium alloy fabricated by squeeze casting

Cyclic deformation behavior and fatigue life of squeeze-cast AZ31 magnesium alloy was studied under stress amplitude-control at room temperature. Low and high cycle fatigue tests with engineering stress amplitudes in the range from 40 to 110 MPa were conducted. Analysis of hysteresis curves was performed. Tension–compression asymmetry of hysteresis loops was not observed; the alloy exhibited cyclic hardening in tension and compression. The fatigue life in the low cycle fatigue region was expressed by Wöhler and derived Manson–Coffin curves. Experimental data in both, the low and high cycle fatigue regions were fitted by means of regression functions. S–N curves exhibited a smooth transition from the low to the high cycle fatigue regions and significant scattering of experimental points was observed. Furthermore, metallographic and fractographic analyses were performed. Crack initiation occurred from the specimen surface or on clusters of secondary particles; the region of final fracture was characterized by a transgranular ductile fracture.It can be concluded that the fatigue properties of squeeze cast magnesium alloy AZ31 are significantly improved comparing to materials prepared by common methods of casting. Squeeze casting also enables the cost-effective fabrication of complicatedly shaped parts.     

纤维对短切碳纤维/AZ91D复合材料热变形行为和加工性能的影响纤维对短切碳纤维/AZ91D复合材料热变形行为和加工性能的影响

采用等温压缩试验研究了不同碳纤维体积分数的镁基复合材料（CFs/AZ91D）和镁合金（AZ91D）在变形温度310~430℃、应变速率10-3~10-1 s-1范围内的塑性变形行为。根据实验结果建立了CFs/AZ91D和AZ91D的热加工图,分析了纤维对CFs/AZ91D塑性加工性能与变形机制的影响。结果表明:相比ZA91D,纤维在提高复合材料流动应力的同时促进了基体动态再结晶和应变软化,但纤维体积分数对流动应力与应变软化程度影响较小,CFs/AZ91D热变形时表现出比ZA91D更高的应变速率敏感指数和变形激活能;ZA91D热加工图不存在变形失稳区且其高温低速率区变形时的能量耗散效率大于30%,CFs/AZ91D高温低应变速率区变形时的能量耗散效率大于50%,此时纤维激励了基体合金动态再结晶而使复合材料表现出极高的能量耗散效率,但在低温高应变速率变形时,基体合金与纤维之间的界面开裂极易导致CFs/AZ91D出现塑性流变失稳行为。      

AZ31 镁合金轧制-剪切-弯曲变形工艺数值模拟研究

目的研究温度和道次压下量的变化对AZ31镁合金轧制-剪切-弯曲变形工艺的影响规律。方法对AZ31镁合金轧制-剪切-弯曲变形过程进行数值模拟,探究变形过程中应力、应变分布规律。结果压下量越大,模具转角处累积的等效应变值越大;随着温度的升高,模具转角处等效应力逐渐减小,等效应变逐渐增大。结论试样在模具转角处发生了剧烈的塑性变形,研究结果为板材的制备提供了依据。     

Effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy under asymmetrical uniaxial cyclic loading

Magnesium alloys are increasingly used in the automotive and aerospace industries for weight reduction and fuel savings. The ratcheting behavior of these alloys is therefore an important consideration. The objective of this investigation was to study the effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy. The experiments have shown that the extruded AZ31B Mg alloy presented the following characteristic behavior with increasing number of loading cycles: first an apparent cyclic softening was observed, then a cyclic hardening occurred, and finally a stable state was reached. This generic behavior can be explained by the fact that the variation trend of the maximum strain with the number of cycles differs from that of the minimum strain. The extrusion ratio did not influence the cyclic softening/hardening behavior or the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the mean stress and the peak stress. However, the extrusion ratio influenced the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the stress amplitude. Increasing the extrusion ratio also reduced the ratcheting strain and the effects of the load history on the ratcheting behavior of the extruded AZ31B Mg alloy.     

Tearing energy of AZ31 magnesium alloy sheets determined by the multiple tensile testing method

Tearing energy of the AZ31 magnesium alloy sheets in the annealed (O-tempered) and half-hard (H24) conditions was studied in both rolling and transverse directions by the multiple tensile testing method. The results showed that while plastic deformation energy was primarily controlled by the strain hardening exponent, tearing energy was directly related to the neck breadth parameter N, which depends on the strain hardening, strain rate hardening, and plastic anisotropy of the tested sheets. It was also found that the tearing energies obtained for the annealed AZ31 sheets were comparable to those of AA5010 aluminium sheets, while the plastic deformation energies were much higher than those of aluminium sheets. This may imply that AZ31 magnesium sheets can be potential candidates for dissipating the impact energy in the vehicles structures which are prone to collision.     

Influences of tensile pre-strain and bending pre-deflection on bending and tensile behaviors of an extruded AZ31B magnesium alloy

This paper focused on the influences of tensile pre-strain and bending pre-deflection on the three point bending and uniaxial tensile properties of an extruded AZ31B magnesium alloy. The influences of pre-strain/deflection on bending/tensile curves could be divided into three stages. The results show that: (1) In the elastic stage, considering the variation of specimen’s cross sectional area, the pre-strain/deflection did not affect the measured elastic modulus obtained from both bending and tensile tests. (2) In the transition hardening stage, the specimen presented obvious hardening behaviors on basis of the pre-strain/deflection, the phenomenon was mainly caused by the strain hardening effects produced from previous uniaxial tensile and bending processes. (3) In the large plastic deformation stage/necking stage, as the accumulation of plastic deformations caused by pre-strain/deflection were significant, the specimen’s ability to resist plastic deformation was weakened. Specially, as the tensile pre-strain increased, the bending load decrement rate gradually decreased, and as the bending pre-deflection increased, both the tensile strength and elongation sharply decreased, the accumulated irreversible plastic work promoted the damage process of the magnesium alloy. The influences of tensile pre-strain on the bending behaviors of the magnesium alloy were also analyzed via finite element method.     

AZ31B镁合金压-压循环载荷下变形行为及变形机制演化EI北大核心CSCD

为探讨AZ31B挤压态镁合金棒材沿径向取样的循环变形特征,开展了0.75%,1.0%,2.0%和4.0%应变幅下应变控制的非对称压-压循环变形实验。结果表明:在小应变幅(0.75%,1.0%)下,循环变形的滞回曲线表现出较好的对称性;在大应变幅(2.0%,4.0%)下,滞回曲线对称性差,且在滞回曲线上出现拐点;随着循环周次增加,塑性应变幅呈现下降趋势,材料均表现出循环硬化行为,在小应变幅下循环拉伸阶段对材料硬化率远大于压缩阶段的硬化率,而在大应变幅下这种区别并不明显。分析表明,沿径向取向的〈1120〉丝织构镁合金,小应变幅下位错滑移在整个寿命周期内作用更大;大应变幅下,随着塑性变形的增加,循环过程中变形机制发生演化,较低临界剪切应力(critical resolved shear stress,CRSS)的基面位错和拉伸孪生不能完全满足变形要求,较高CRSS滑移系启动及残余孪晶使得滞回曲线出现拐点;循环变形过程中不完全的孪生-去孪生过程使基体中存在大量残余孪晶,影响了循环变形过程的硬化率,同时降低了疲劳寿命。     

Time-dependent uniaxial behavior of rolled magnesium alloy AZ31B at 393 K and room temperature

To address the time-dependent properties of rolled AZ31B alloy, we conducted typical tests of the rate jump, creep, and stress relaxation at room temperature and 393 K. In the rate jump tests, the tensile curve exhibited a strong dependence on the strain rate, whereas compression was totally insensitive to the stress rate at both temperatures. For the creep and stress relaxation test, we observed creep strain and decay stress in the compression, which was weaker than the tensile curve. The plastic viscosity increased at 393 K because the dislocation motion was thermally activated. We then applied thermal activation theory for the repeated stress relaxation tests. The activation volume implies that cross-slip and dislocation nucleation are the operating mechanisms for creep and stress relaxation.

     

Cyclic hysteresis of AZ31B extrusion under load‐control tests using embedded sensor technology

The fatigue behaviour of AZ31B extrusion magnesium alloy under load‐control cyclic test conditions is estimated using a combination of simulation and experimental results. The strain measurement of this asymmetric material is found experimentally using a Fibre Bragg Grating (FBG) sensor during rotating bending tests. Then, to analyse applied stresses in the sample – particularly in the plastic deformation range – the Variable Material Property (VMP) method is employed. Using this simulation method, the hysteresis loops of two critical top and bottom elements of the sample's cross section under different bending moments are obtained. Finally, the strain of the sample during rotating bending, as measured by the embedded FBG sensor, is related to the stresses obtained from the modeling using a mapping function. The hystereses obtained from this combination of the modeling and experimental results are compared with the results of a companion strain‐control pull–push test in which the input strain history was that of measured by the FBG sensor. Observations verify that the stresses of the combined VMP‐FBG hysteresis loops have good compatibility with the stress responses obtained through the experiment. The hybrid model introduced in this work can be employed to capture cyclic hysteresis, and hence estimate the fatigue life, under load‐controlled rotating bending tests.